Motor KV doesn't make sense ??

[john haz]

Hi Chaps

 

First off i pulled out the standard 30A ESC (that is going into my sabre) & replaced it with a 60A

 

popped it all together & tested (all standard) & got 282w on the standard motor, which seems pretty reasonable (original ESC gave 276w so probably only a "reading" or battery difference ?)

 

But .... being a "twiddler" & having another motor sitting here gathering dust, i decided to change it

 

The original installed inrunner is a 4300kv (according to GP's site). I fitted a 5000kv Turnigy THIS which is a 5000kv, rated @ 33 amps max.

 

Doing simple maths, if the 4400kv was pulling 27A, the 5000 should pull around 30 giving me just over 300w for a smooth hand launch

 

NO WAY !! half throttle gave me 335w Any more than half throttle pulled so much, the ESC was hitting LVC !!

 

Now i know i was cutting it fine with the motor, but WHY was it pulling so much power at half stick ??

 

Refitted the original & at half stick it showed about 160w (give or take a bit), so the throttle appears fairly linear.

 

It doesn't make sense does it ???

 

 

 

** None of the is too important, just me being inquisitive. I have just ordered a new 49A 4800kv inrunner HERE which i hope will give me 350 - 400 on my 40c 2200's (and the reviews say it can handle 4S, so there's an option to make it a rocket )

[Tim Mackey]

What simple maths ?

[Simon Chaddock]

john

If we assume the kV ratings are correct are you using the same prop or fan?

 

The problem is the power required to turn a prop/fan rises at least as the square of the RPM.

So you increased the revs by 5000/4300 = 1.16 but that will draw at least 1.16 x 1.16 = 1.34 times the current.

 

On this basis the result you obtained is not so surprising.

Edited By Simon Chaddock on 16/05/2011 18:19:25

[Tom Wright 2]

The difference in kv is 700 KV so if you are using say a 4 cell that's 14 v x 700 kv = 9800rpm more than the lower kv motor ,rather self explanatory Simon its goingto draw loads more current.

Edited By tom wright 2 on 16/05/2011 18:24:07

[john haz]

Ah! Simon, it looks as if you've hit the nail on the head

 

To answer your post and Tim's question....

 

Same everything, ESC, fan, job lot.

 

i assumed that (give or take a bit for the load), the KV would increase the watts / load in a proportional fashion, obviously not !

 

My "simple maths" was

 

4300kv = 280w

 

280/43*50 = 325w

(280w original / 4300kv * 5000kv)

 

Simples ..... well, obviously not !

 

i am learnig though Peeps Now i know how to calc it roughly - gonna order meself a bucket of squares

[john haz]

Everything tested on 3s Simon

 

Now if i am capable of understanding the correct simple maths,the old motor was

 

 

 

4300kv @ 280w

 

the new inrunner 4800kv should give me

 

4800/4300 = 1.116279 squared =1.246

 

280w * 1.246 = 350w

 

just what i am looking for

 

 

(or 560+ on 4s ?) 

 

 

 

 

 

 

Edited By john haz on 16/05/2011 18:43:28

[Tim Mackey]

[john haz]

A from the sticky bun corner is a result

[Tom Wright 2]

Posted by tom wright 2 on 16/05/2011 18:22:48:

The difference in kv is 700 KV so if you are using say a 4 cell that's 14 v x 700 kv = 9800rpm more than the lower kv motor ,rather self explanatory Simon its goingto draw loads more current.

Edited By tom wright 2 on 16/05/2011 18:24:07

Sorry quick post tea was ready !

 

Should have been addressed to John .THE RULE...... kv up prop down.

 

Tom. 

Edited By tom wright 2 on 16/05/2011 19:08:01

[paul (dig-it-up)]

uugghh all double dutch to me lol . iawnski will be along in a minute to explain to me the maths wont you kevin

[iawnski]

head in sand

[john haz]

Tom - not easy to "prop down" with edf though is it

 

i wanted more power, that was the idea behind increased kv, but it seems i overcooked it a bit LOL

 

Wheres that ostrich gone !

[Tom Wright 2]

Hi John.

Yes of course i know that ,but the simple V x KV sum gives a simple and useful comparison that even an ostrich can understand LOL. And the watt meter does the rest.

Just imagine what happens when a watt meter is not used   .

 

Cheers

Tom.

Edited By tom wright 2 on 16/05/2011 20:06:29

Edited By tom wright 2 on 16/05/2011 20:08:45

[Malcolm Fisher]

A Watt meter seems to be a mandatory piece of kit for electric motor operation. At present I only have a couple of brushed motors running on direct current and my searches so far have been unable to find a Watt meter which will work on DC.

My small multimeter is too fragile to measure the currents drawn and would burn out if incorporated into the circuit for more than a few seconds.

Have I missed something somewhere? Can anyone point me in the direction of at least an ammeter sufficiently robust to do the necessary measurements?

I am loath to abandon the power plants I have while they are still performing adequately but can't do any experimenting with propeller sizes or different batteries without the risk of ruining my ESCs and/or motors. I know that I will eventually have to take the brushless route or get out my IC power models once again.

Malcolm

[john haz]

Believe it or not Tom, theres a few lads at our local field that fly VERy well, have been doing it for years & have never used a meter on anything !

 

I guess it's a bit of trial & error, a bit of experience & a dose of luck for them, but they seem to get it right.

 

I like metering stuff, playing with different motors / props / fans & learn a bit in the process.

 

Each to his own

[Tom Wright 2]

Thanks John.

One things for sure ,when we stop learning ,we have lost interest.Or brains are full !

Know what you mean about the none w meter users ,i can look a the size and KV rating and usually predict a suitable prop , but there are loads on here looking in who don't have that experience ,and need a safe way to the more advanced learning curve.

 

Cheers

Tom.

[PatMc]

Posted by Malcolm Fisher on 16/05/2011 20:28:31: Can anyone point me in the direction of at least an ammeter sufficiently robust to do the necessary measurements?

Here's a suitable meter I've no experience of the particular one but I assume it will give current & voltage readings as well as Watts.

A search around the likes of BRC, Robotbirds etc will turn up plenty more options.

[Tom Wright 2]

Posted by Malcolm Fisher on 16/05/2011 20:28:31:

without the risk of ruining my ESCs and/or motors. I know that I will eventually have to take the brushless route or get out my IC power models once again.

Malcolm

Gosh Malcolm we nearly lost you back to the IC crowd ,Pat Mc saves the day......i hope .

Tom.

[Malcolm Fisher]

Tom, I wouldn't be "lost back to the IC crowd", because I've never left it

Pat Mc's information is very useful, Thank you Pat.

Having gone back to more slope soaring though I have dipped my toes into the electric pool as well - mainly because I have a fairly long round trip and on some days the gliders I had were unsuitable for the conditions. I use electric motors as an "old man's tow line" so that I can get some therapy on windless, liftless days.

I must admit that I could become hooked but will never totally abandon IC and still, on occasion, use a small diesel power pod in the same role of old man's tow line. There is something very addictive about diesel power plants...

Malcolm

[Tom Wright 2]

Know what you mean Malcolm i can still remember the sounds and smells of the mills , ED Racer ,heron ,PAW etc and that was 50 years ago.

Hope you do get some brush-less motors the difference is amazing,but what ever takes your fancy enjoy your flying...... we are off thread now  .

 

Tom

Edited By tom wright 2 on 16/05/2011 22:16:43

[John Privett]

Posted by PatMc on 16/05/2011 20:45:59:

Posted by Malcolm Fisher on 16/05/2011 20:28:31: Can anyone point me in the direction of at least an ammeter sufficiently robust to do the necessary measurements?

Here's a suitable meter I've no experience of the particular one but I assume it will give current & voltage readings as well as Watts.

A search around the likes of BRC, Robotbirds etc will turn up plenty more options.

 

That's precisely the one I have Pat. It does exactly what you say - and even provides a variable "throttle" input to the ESC if you want it to.

 

It's dropped in price a little since I bought mine too! (Got mine at the Longhorsley show a year or two ago)

[PatMc]

John, I have a first version Emeter which displays Volts, Amps, Watts, RPM & has 5 memory storage plus a number of other functions. It was pricy at the time but the newer version is even more now albeit with more functions.

TBH if I didn't have it I'd get a clamp ammeter, use my old tach & not bother much about noting the Watts.

 

Malcolm, is that the 100S Gold Cloud in your photo gallery ?

 

[Malcolm Fisher]

Hello Pat,

It is. You have sharp eyes and a good memory. The Gold Cloud was bought from a former club member who was having a clearout. I bought it (and a Black Magic) at a good price because I liked the look of it and also because it featured airbrakes which I wanted to try. These turned out to be spoilers rather than brakes and there was no material in the kit to make them. They have though, proved effective and helped me get it back on the groud where and when I want.

The kit langushed in my shed for several years before I got the necessary "roundtooit".

The build was an interesting exercise and definitely not a beginner's model. I took pictures of the build at most stages intending to wriie a sort of review. I've done this but it needs polishing and the images need captions.Will try to get round to submitting it to the magazine or on here but don't hold your breath.

Malcolm

[PatMc]

Hi Malcolm,

I remember it because I've got an unfinished kit that's in need of a "roundtooit". I also picked it up at a good price several years ago, the wing inner panels & tailplane having already been nearly completed.

It will either get fitted out with an electric motor or I may use the flying surfaces on a spare glider fuselage. The wing panels have the outline parts for the spoilers & plastic guide tubes for the lines fitted but I'll fit micro servos instead. I'm also considering reducing the tip dihedral & having ailerons.

Cheers,

Pat.

[Malcolm Fisher]

Hi Pat,

I found the ply fuselage sides to be very poorly cut and they fitted where they touched - not much gluing area on 1/16" ply. Much of the precut balsa didn't match the plan. Fortunately all were oversize so it was an irritation rather than a problem.

I considered adding ailerons but decided making the spoilers was enough complication and anyway I wanted to build it as designed.to give a relaxing model which it really is. It can also cope with quite a range of wind speed when flown off the slope.

Malcolm